Adjustments (known as the "Centurion Leap Day Rule" ie div.4/skip100th/count400th or div.4/skip700th/count2800th)

Further adjustments that are necessary because an average year is actually 365.2425 days rather than 365.25 days. Do you see where the problem is? We "over-rounded" by adding a leap day in every 4 years. Hence, we have to do more "subtractions". (Leap days) Adjustments to Gregorian calendar:365.25 - 365.2425 = .2425 days.25 - .2425 = .0075.0075 * 400 years = 3 days

(The most confusing part is that we were adding leap-days every 4 years to a 365-day calendar and we were applying the further reverse adjustments of .25 - .2425 = .0075 * 400 years = 3 days. But now we are adding leap-week every 7 years (double-leap-week every 28 years) to a 364-day calendar and we are now applying the further reverse adjustments of .25 - .2425 = .0075 * 2800 years = 21 days.)